Casing rotary steerable system for drilling

ABSTRACT

A system for drilling a deviated borehole, comprises a drill string, a bending mechanism supported on the drill string, a drill bit supported on the bending mechanism such that the axis of the bit does not align with the axis of the drill string where it connects to the bending mechanism, and a liner supported on at least one of the bit and the drill string and having upper and lower ends, wherein the bending mechanism is disposed within the liner such that the upper and lower liner ends are not co-axial.

RELATED CASES

Not applicable.

FIELD OF THE INVENTION

The invention relates to a casing-drilling system with directional capabilities. More particularly, the present invention relates to a system in which a bending mechanism supports the drill bit and extends through a casing or liner such that the casing or liner is bent by the bending mechanism.

BACKGROUND OF THE INVENTION

In oil and gas drilling operations it is often desirable to change direction in the course of drilling. Generally, the goal is to change the direction in which the drill bit at the lower end of a drill string drills, so that it does not drill along the central longitudinal axis of the lower part of the drill string. Many drilling systems and methods have been developed for this purpose. These include “point-the-bit” systems, in which the axis of rotation of the drill bit is deviated from the axis of the bottom hole assembly (BHA), such as by including a bend in the BHA itself, and “push-the-bit” systems, in which eccentric stabilizers or eccentric actuators push the bit into a desired orientation with respect to the axis of the BHA.

Oil and gas wells are typically drilled using a drill string that supports and propels the bit. After a desired length of hole has been drilled, the drill string is typically removed and a casing or liner is cemented into the hole. “Liner” refers to casing that is set below a preceding length of casing. Unless expandable tubulars are used, a liner will have a smaller diameter than the casing or liner above it. Liners are typically suspended from the upper string by a hanger device. For purposes of the following discussion, casing and liner are considered interchangeable.

Because of the significant time and effort required to pull the drill string out of the hole and place and set a length of casing, a modification of conventional drilling has been developed, in which a length of casing is used as the drill string. In this type of operation, termed “casing drilling,” the bit is mounted at the lower end of the casing string and rotation of the bit is provided by rotating the casing string. Once the desired depth has been reached, the bit is disconnected from the casing and retrieved through the casing to the surface, whereupon the cycle can be repeated. In some instances, the bit may be directly attached to the casing string and left in place. In these cases, the bit is typically drilled out by the next smaller sized bit.

When it is desired to combine the concepts of directional drilling and casing drilling, various aspects of the operation make it difficult to combine the mechanical systems that accomplish each, as mentioned in U.S. Pat. No. 7,086,485. It is therefore desired to provide a system that effectively allows directional casing drilling without requiring cumbersome or expensive equipment downhole.

SUMMARY OF THE INVENTION

In accordance with preferred embodiments of the invention there is provided a system for drilling a deviated borehole, comprising: a drill string, a bending mechanism supported on the drill string, a drill bit supported on the bending mechanism such that the axis of the bit does not align with the axis of the drill string where it connects to the bending mechanism, and a liner supported on at least one of the bit and the drill string and having upper and lower ends, wherein said bending mechanism is disposed within said liner such that the upper and lower liner ends are not co-axial.

The bending mechanism preferably contacts the inside of the liner at at least three axially spaced-apart locations, and may include a stabilizer. The system may include at least one sleeve surrounding the bending mechanism such that said sleeve transfers a bending force to the liner. The lower end of the liner may be attached to the bit and the liner may rotate with the bit.

As used in this specification and claims the following terms shall have the following meanings:

With respect to a borehole, “above” shall refer to positions that are relatively closer to the surface and “below” shall refer to positions that are relatively closer to the bottom of the borehole, even if the borehole is deviated or horizontal.

As mentioned above “casing” and “liner” may be used interchangeably. Thus, references to “casing drilling” or “liner drilling” are intended to apply equally to both types of drilling.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more detailed understanding of the invention, reference is made to the accompanying Figure, which is a schematic illustration of a system constructed in accordance with the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to the Figure, a casing drilling system 10 in accordance with a preferred embodiment extends from the surface (not shown) into a borehole 12 in a formation 14. A portion of borehole 14 above casing drilling system 10 may be cased with a previously-placed casing string 16. Casing drilling system 10 includes a drill string 18, a liner string 20, a mandrel 22, upper and lower latches 30, 32, respectively, an eccentric bearing 34, and a bit 24. In other embodiments, bit 24 may be supported on a mandrel (not shown).

In the embodiment shown, during drilling, torque is applied to drill string 18 at the surface, transmitted from drill string 18 to liner string 20 at the top of liner 20, and from liner 20 to bit 24. In some instances, a liner hanger running tool 25 may be included between the liner and the drill string, so that torque is applied to the bit via a path that includes drill string 18, liner hanger running tool 25, and liner 20, as indicated by arrows 27, 28, and 29. Alternatively, torque can be transmitted from drill string 18 to bit 24 at the bottom of liner 20. In still other embodiments (not shown), torque can also be transmitted from the drill string to a torque sleeve, from the sleeve to the liner, and from the liner to the bit. In the discussion that follows, it will be understood that the concepts relating to the invention are not dependent on the mechanism by which torque is transmitted to the bit. It will be understood that mud motors, such as are known in the art, may be used as another source of torque and in some instances could cause the bit to rotate faster than the liner.

Regardless of the path, torque is ultimately transmitted to bit 24, causing bit 24 to rotate and advance through formation 14. Liner string 20 advances with the bit.

Liner string 20 and bit 24 are preferably connected by a releasable mechanism that allows torque to be transmitted from liner string 20 to bit 24 during drilling and also allows bit 24 to be disconnected from liner string 20 when the desired hole depth has been reached. An example of a suitable mechanism for this function is sold under the name “E-Z Case” by Hughes Christensen Company of Houston, Texas. As mentioned above, in other embodiments, the bit may be left in place. Similarly, upper and lower latches 30, 32 and eccentric bearing 34 are supported in the liner in a manner that allows them to be retrieved.

In order to drill a deviated hole, it is necessary to apply a lateral force that deflects the bit, so that the bit cuts along a path that is different from the axis of the hole immediately above the bit. As mentioned above, in non-casing-drilling systems, push-the-bit and point-the-bit systems rely on mechanisms that push against the formation so as to apply an opposing force to the bit, forcing its axis to differ from the main tool axis.

According to preferred embodiments of the present invention, a lateral force is applied to the bit by a three-point system that is contained within liner string 20. The three-point system may include upper latch 30, lower latch 32, and eccentric bearing 34. As shown in the Figure, upper and lower latches 30, 32 may hold mandrel 22 in a central position within liner string 20, while eccentric bearing 34 has an off-center bore through which drill string 18 passes. The eccentricity of the bearing is illustrated by arrows 44, 46, which are unequal in length.

Unlike liner string 20, bearing 34 does not rotate relative to the formation. Instead, it is provided with means that cause it to rotate around mandrel 22 in a direction opposite to the direction of rotation of liner string 20, as indicated at 37. Such a device is preferably an actively powered device that is capable of sensing its position and can apply counter-rotation to the eccentric bearing 34, allowing it to remain substantially motionless with respect to the borehole. Such devices are known and are commercially available.

For example, rotation of bearing 34 can be provided by an actively powered device applying counter-rotation to the bearing 34, thereby allowing it to remain rotationally motionless with respect to the earth. One example of such a device is a hydraulic power unit. In contrast, latches 30 and 32 can rotate in a 1:1 ratio with liner string 20.

Because of the off-center support provided by eccentric bearing 34, mandrel 22 tends to be pushed toward one side within liner 20. The competing forces exerted by eccentric 34 bearing and latches 30 and 32 cause mandrel 22 to bend in one direction and liner 20 to bend in an opposite direction. The degree bending of each component will depend on the relative stiffnesses of mandrel 22 and liner 20. If mandrel 22 is sufficiently stiff relative to liner 20, liner 20 can be bent by a desired amount, with the result that both the distal end of liner 20 and bit 24 will be pointed in a desired direction.

Because eccentric bearing 34 does not rotate relative to formation 12, the direction in which bit 24 is pointed will remain substantially constant. As liner string 20 rotates, it will undergo cyclic or periodic deflection. For this reason, it may be desirable to consider material selection and expected stress levels when designing the components of the system, as is known in the art. Thus, in one exemplary embodiment, liner string 20 comprises a composite material with a low transverse stiffness, while mandrel 22 comprises a relatively stiff material, such as beryllium, aluminium-beryllium composites, beryllium titanium composites, and the like.

It will be understood that the eccentric bearing 34 can be replaced with a set of selectively retractable pads (not shown) that periodically extend and retract as the liner rotates and serve to keep the eccentricity of the bending mechanism oriented to a constant azimuth.

If desired, one or more stabilizers 40 may be provided at various points along the liner string. Stabilizers 40 serve to center the equipment in the borehole and reduce movement of the tool during drilling. In preferred embodiments, at least one stabilizer is included near the upper end of liner string 40. It is preferred that stabilizers 40 be close to upper latch 30 so as to maximize the effect of the bending.

It will be understood that the preceding discussion has described the concepts of the present invention with respect to certain preferred embodiments and that variations in the components of the system can be varied without departing from the scope of the invention, which is set out in the claims that follow. 

1. A system for drilling a deviated borehole, comprising: a drill string; a bending mechanism supported on the drill string; a drill bit supported on the bending mechanism such that the axis of the bit does not align with the axis of the drill string where it connects to the bending mechanism; and a liner supported on at least one of the bit and the drill string and having upper and lower ends, wherein said bending mechanism is disposed within said liner such that said upper and lower liner ends are not co-axial.
 2. The system according to claim 1 wherein the bending mechanism contacts the inside of said liner at at least three axially spaced-apart locations.
 3. The system according to claim 1 wherein the bending mechanism includes a counter-rotating device that maintains the orientation of the bending substantially fixed relative to the borehole.
 4. The system according to claim 1 wherein the bending mechanism includes a mandrel that is stiffer than the liner.
 5. The system according to claim 4 wherein the mandrel is constructed of a material that comprises beryllium.
 6. The system according to claim 1 wherein the lower end of the liner is attached to the bit and the liner rotates with the bit.
 7. The system according to claim 1 wherein the upper end of the liner is attached to the drill string.
 8. The system according to claim 1, further including a stabilizer disposed between the liner and the borehole. 